During the installation of offshore platforms or similar structures, a set of pile grippers is typically utilized to secure a platform to the ocean floor. FIG. 1 illustrates an offshore platform with a deck 100 above water surface 106 and the deck 100 is supported by extended jacket legs 102 to the sea floor 105. There is a plurality of skirt pile sleeves 104, each for housing one driven pile 103 through the middle of the sleeve. A plurality of pile grippers 108, typically one gripper 108 per jacket corner leg 102, are installed at a corresponding sleeve 104 top and below a stabbing guide 107. When activated, the pile gripper 108 mechanically grips the driven pile 103 through a plurality of hydraulic cylinders and locks the offshore platform through the corresponding sleeve 104 to the ocean floor 105. Typically, the grippers 108 need to be activated/engaged and deactivated/released several times during a jacket leveling operation before grouting. After grouting, the piles 103 and sleeves 104 are permanently fixed to each other and then all the pile grippers 108 are released.
A conventional pile gripper of prior art comprises a plurality of hydraulic cylinders evenly spaced and circumferentially mounted in a steel can and then welded to a jacket leg or a skirt pile sleeve. These hydraulic cylinders are usually powered by a hydraulic pump operated at the surface of an offshore platform and are connected via a supply line to each gripper assembly near the ocean floor. These hydraulic grippers can also be operated by ROV or via diver intervention. As described above, a mechanical lock can be activated by applying hydraulic pressure via cylinders forcing a front head of each cylinder, which has a head plate with tooth rows, towards the driven pile. Once contact is made between the pile outer surface and the cylinder head's teeth, the cylinder front head deforms the pile outer surface locally around the point of contact in order to perform the gripping action. In short, a conventional pile gripper needs to have high gripping power, to be relatively small in cylinder size with high internal pressure and a relative short stroke, to be resistant to seawater corrosion and, above all, to have high overall system reliability. However, the required stoke distance for each cylinder is typically limited.
A Conventional Pile Gripper
FIG. 2A illustrates an ISO cut-off view of a conventional pile gripper 108 with a steel can, whose wall is thicker than a sleeve 104 wall below, and with a plurality of evenly spaced hydraulic cylinders 110 circumferentially mounted and fixed in the steel can and at the top of a sleeve 104 below a stabbing guider 107 and with a control assembly 116 attached. A driven pile 103, with rows of shear keys at pile top outer surface, is placed through the middle of the sleeve 104 with a gripping mechanism. There are tooth rows 117 at the surface of the front head plate 125 of each cylinder 110 and there are a pair of hydraulic fluid lines 118, 119, for each cylinder 110, 119 for pushing the cylinder head plate inward and 118 for retracting the cylinder head plate 125 backward.
FIG. 2B illustrates the top view of the evenly spaced hydraulic cylinders 110 circumferentially mounted at the gripper 108 steel can, without the control assembly 116, in an engaged configuration with cylinders 110 extended and the teeth 117 contacting the driven pile 103 outer surface for a gripping action.
FIG. 2C illustrates the cut-off section view from FIG. 2B with the evenly spaced hydraulic cylinders 110 in the gripper 108 and with teeth 117 from each extended hydraulic cylinder front head plate 125 surface.
A Conventional Hydraulic Cylinder Used for Pile Gripper
FIG. 3A illustrates a cross section view of a conventional hydraulic cylinder 210, fixed in a steel can (not shown) and used for a pile gripper (not shown), comprising a piston 222, a piston rod 223 disposed within a barrel 228, and a circular front head plate 225 with tooth rows 217 at its front. A sliding O-ring seal 221 and a wiper 220 are installed in the barrel 228 with an end cap plate 226 attached to form sealed chambers 224/234 and a stopper 239 to limit the maximum stoke of the cylinder 210. The sliding seal 221 and the wiper 220 act to seal hydraulic fluid in the barrel 228 while permitting extension and retraction of the piston rod 223 with respect to the barrel 228. During an extension operation, hydraulic fluid 229 is pumped into the back chamber 224 through the back line 219, thus forcing the piston 222 forward. There are two types of retraction operation, as in a single-acting cylinder vs. a double-acting cylinder. During a single-acting cylinder's retraction operation, the piston is forced backward by a built-in spring. During a double-acting cylinder's retraction operation, hydraulic fluid 229 is pumped into the front chamber 234 through the front line 218 and, at the same time, the same amount of hydraulic fluid is then pushed out of the back chamber 224 through the back line 219.
FIG. 3B illustrates the section view of the conventional hydraulic cylinder 210, shown in FIG. 3A, in a maximum extended position. Prior to an extension operation, all chambers 224/234 inside the barrel 228 shall be full of hydraulic fluid 229. During the extension operation, hydraulic fluid 229 is pumped into the back chamber 224 through the back line 219, while the same amount of hydraulic fluid 229 is pushed out of the front chamber 234 through the front line 218. The increased internal pressure will push the piston rod 223 forward and make a maximum stroke distance L1 for the front head plate 225 with the teeth 217 at its front surface.
FIG. 3C illustrates the section view of a conventional double-acting hydraulic cylinder 210, shown in FIG. 3A, in a fully retracted position. During the retraction operation, hydraulic fluid 229 is pumped into the front chamber 234 through the front line 218 and, at the same time, the same amount of hydraulic fluid is then pushed out of the back chamber 224 through the back line 219.
Conventional hydraulic cylinders are widely employed in almost all industries including offshore industry. Conventional hydraulic cylinders, however, have some inherent disadvantages. Firstly, their fabrication cost is high, which accounts for the lion's share of a pile gripper's overall cost. Such high cost is closely related to the requirement of strict tolerance on precision machining. In addition, the fluid employed in hydraulic cylinders is usually an oil derivative and, therefore, expensive. In the application of submerged pile grippers, a large quantity of hydraulic fluid will be needed especially for deepwater application because of the long supply lines. Secondly, these cylinders are water depth dependent because the chamber pressure is always sealed off from the outside surroundings, and so the deeper into the sea, the higher the water pressure to be overcome. As water depth increases, the required internal pressure has to be increased accordingly, thus causing a considerable cost impact, as the cost of these cylinders is sensitive to the pressure increase. Thirdly, the hydraulic fluids can, however, be an environmental contaminant, in case of leakage, particularly when large quantities are used.
It is, therefore, desirable to provide a new type of hydraulic cylinder used for a pile gripper which does not employ pistons or sliding seals or rings, and therefore such cylinders can be manufactured with less strict tolerance at a lower cost. It is also desirable to provide a system that can employ inexpensive and environmentally friendly fluids, such as fresh water or seawater. It is further desirable to provide an active fluid power system with a built-in automatic retraction mechanism to eliminate the need for two fluid lines and two chambers as in the case of a double-acting cylinder. In short, an ideal new generation cylinder will need to be as powerful as, or even more powerful than, conventional cylinders at a lower cost but with higher reliability.